Optimization of a dynamic absorber with nonlinear stiffness and damping for the vibration control of a floating offshore wind turbine toy model

Passive vibration mitigation of offshore wind turbines using nonlinear absorbers or nonlinear energy sinks has started to receive attention in the literature. In most cases, little attention has been paid to the possibility of detached resonances that occur when the nonlinear energy sink is attached to the linear system describing the wind turbine. Sea motions that alter the initial conditions of the floating offshore wind turbine may cause the nonlinear energy sink to operate at one or more detached resonances, completely negating its ability to control turbine vibration. In this paper, we are interested in optimizing the parameters of a nonlinear energy sink with nonlinear stiffness and nonlinear viscous damping for vibration control of a toy model (e.g., a linear mass-spring-damper system) of a floating offshore wind turbine over its entire operating range. The mechanism of cancellation of the detached resonance is studied analytically under 1:1 resonance. It is shown that the nonlinear energy sink with properly tuned nonlinear viscous damping allows the complete elimination of undesired regimes and completely restores the absorber's ability to strongly limit the vibration of a floating offshore wind turbine over its entire forcing range. The results obtained over a wide range of parameters suggest that both the optimal nonlinear energy sink parameters (linear and nonlinear stiffness and nonlinear damping) and the damping of floating offshore wind turbine vibration depend on simple power laws of nonlinear energy sink mass and linear damping.